High-pressure threaded union with metal-to-metal seal, and metal ring gasket for same

ABSTRACT

A metal ring gasket provides a high-pressure temperature tolerant metal-to-metal seal between subcomponents of a threaded union. The metal ring gasket is received in an annular cavity formed between mating surfaces of the subcomponents of the threaded union. The metal ring gasket is capable of maintaining a fluid seal even at very high temperatures resulting from direct exposure to fire. At high fluid pressures the metal ring gasket is energized because hoop stress induced by the fluid pressure forces the metal ring gasket into tighter contact with the subcomponents of the threaded union.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is the first application filed for the present invention.

MICROFICHE APPENDIX

Not Applicable.

TECHNICAL FIELD

The present invention relates generally to sealed joints forhigh-pressure fluid conduits and, in particular, to a metal ring gasketfor threaded unions for use in very high fluid pressure applications.

BACKGROUND OF THE INVENTION

Threaded unions are used to provide fluid-tight joints in fluidconduits. Threaded unions are held together by a threaded nut that istightened to a required torque using a hammer or a wrench. In the oilindustry, threaded unions are generally constructed using “wing nuts”and are commonly called “hammer unions” or “hammer lug unions”. Hammerunions are designed and manufactured in accordance with thespecifications stipulated by the American Petroleum Institute in API 6Aentitled “Specification for Wellhead and Christmas Tree Equipment”.Hammer unions are usually available in a variety of sizes (1″ to 12″)and a variety of pressure ratings (1000 psi to over 20,000 psi).

One substantial disadvantage of most prior-art threaded unions is thatthey rely on elastomeric seals for achieving a fluid-tight joint.Elastomeric seals are vulnerable to the extreme temperatures generatedby fire. In the event that a fire erupts around a high-pressure conduit,the elastomeric seal in the threaded union may leak or fail completelywhich may exacerbate the fire if the leak permits combustible fluids toescape to the atmosphere.

While flanged unions are commonly used in well trees, pipelines andother high-pressure applications where temperature tolerant seals arerequired, flanged unions are relatively expensive to construct andtime-consuming to assemble in the field. Metal ring gaskets are knownfor flanged unions, such as the BX ring gasket manufactured inaccordance with API 6A. In operation, however, these BX ring gaskets aredeformed beyond their yield strength and must be discarded after asingle load cycle.

It is well known in the art that there is increasing pressure on the oilindustry to produce hydrocarbons at a lower cost. Consequently, aninterest has developed in utilizing wellhead equipment that is lessexpensive to construct and is more quickly assembled than prior artflanged unions. Threaded unions provide a good alternative to flangedunions from a cost standpoint because they are faster to assemble andless expensive to construct. However, due to safety concerns related tothe lack of a reliable high-pressure metal-to-metal seal, use ofthreaded unions for well tree components and other high-pressuretemperature tolerant applications has not been endorsed.

Therefore, it is highly desirable to provide an improved threaded unionhaving a high-pressure metal-to-metal seal.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved threaded union for providing a high-pressure, fluid-tight,metal-to-metal seal.

The invention therefore provides a threaded union for providing ahigh-pressure, fluid-tight, metal-to-metal seal in a fluid conduit, thethreaded union comprising: first and second subcomponents that areinterconnected by a nut, the first and second subcomponents havingrespective mating ends with complementary ring gasket grooves thereinthat form an annular cavity when the mating ends abut; and a metal ringgasket received in the annular cavity, wherein the metal ring gasket hasan outer diameter that is slightly larger than an outer diameter of theannular cavity and the metal ring gasket is elastically deformed in theannular cavity to provide a high-pressure fluid-tight seal between thefirst and second subcomponents when the first and second subcomponentsare securely interconnected by the nut.

The invention further provides a metal ring gasket for providing ahigh-pressure, fluid-tight metal-to-metal seal in an annular cavityformed by annular grooves at an interface of first and secondsubcomponents of a threaded union, the metal ring gasket comprising agenerally annular body having beveled outer corners for receivingcompressive loads exerted on the metal ring gasket by complementarysurfaces on an outer diameter of the annular cavity when the first andsecond subcomponents are securely interconnected, the metal ring gaskethaving an outer diameter that is slightly larger than an outer diameterof the annular cavity and the metal ring gasket is elastically deformedin the annular cavity to provide a high-pressure fluid-tight sealbetween the first and second subcomponents when the first and secondsubcomponents are securely interconnected by the nut.

The invention further provides a method of providing a high-pressurefluid-tight seal between first and second subcomponents of a threadedunion, the method comprising: determining an inner diameter and an outerdiameter of an annular metal ring gasket groove in a mating face of thefirst and second subcomponents; and manufacturing a metal ring gasket tobe received in an annular cavity formed by the respective metal ringgasket grooves when the first and second subcomponents are securelyinterconnected by a nut of the threaded union, the metal ring gaskethaving outer faces for mating contact with complementary faces in therespective metal ring gasket grooves, an outer diameter that is slightlylarger than an outer diameter of the annular ring gasket grooves and aninner diameter that is slightly larger than an inner diameter of thering gasket grooves so that the metal ring gasket is elasticallydeformed when placed in the annular grooves and the first and secondsubcomponents are securely interconnected, but a gap remains between aninner side of the metal ring gasket and an inner surface of the annularcavity.

The threaded union in accordance with the invention can be used toconstruct wellhead components, well tree components, or joints in anyfluid conduit where a reliable high-pressure temperature tolerant fluidseal is required.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the appended drawings, in which:

FIG. 1 is a cross-sectional view of a threaded union and a metal ringgasket in accordance with one embodiment of the invention;

FIG. 2 is an exploded, cross-sectional view of the threaded union shownin FIG. 1;

FIG. 3 is a cross-sectional view of a threaded union and a metal ringgasket in accordance with another embodiment of the invention;

FIG. 4 is a cross-sectional view of the metal ring gasket shown in FIGS.1-3 immediately prior to elastic deformation of the metal ring gasket asthe threaded union is tightened to a sealed condition;

FIG. 5 is a cross-sectional view of the metal ring gasket shown in FIGS.1-3 schematically illustrating an extent of elastic deformation of themetal ring gasket when the threaded union is in the sealed condition;

FIG. 6 is a cross-sectional view of the metal ring gasket shown in FIGS.1 and 3 when the threaded union is in the sealed condition and underelevated fluid pressure; and

FIG. 7 is a cross-sectional view of the another embodiment of a metalring gasket in accordance with the invention in a sealed condition underelevated fluid pressures.

It should be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention provides a threaded union with a metal ring gasket thatprovides a high-pressure, temperature tolerant, metal-to-metal fluidseal between a first subcomponent and a second subcomponent of thethreaded union. The metal ring gasket is made of ductile carbon steelfor non-corrosive fluid service or ductile stainless steel for corrosivefluid service. The metal ring gasket has outer beveled corners and isreceived in a beveled annular groove in a mating end of the firstsubcomponent. When compressed between the first and the secondsubcomponents, the metal ring gasket deforms elastically to provide anenergized high-pressure fluid seal. The high-pressure seal is capable ofcontaining fluid pressures of up to at least 30,000 pounds per squareinch (psi), and is not affected by elevated temperatures below a meltingpoint of the ductile steel of the metal ring gasket.

Throughout this specification, the terms “first subcomponent” and“second subcomponent” are meant to denote any two contiguous componentsof a joint in a fluid conduit that are joined together using a threadednut.

FIG. 1 illustrates a threaded union 10 in accordance with an embodimentof the invention. The threaded union 10 includes a first subcomponent 12and a second subcomponent 14. The first and second subcomponents 12, 14are generally annular bodies that are interconnected to define a centralfluid passageway 15 as part of a high-pressure fluid conduit. The firstsubcomponent 12 has a mating end 16 that abuts a mating end 18 of thesecond subcomponent 14. The first subcomponent 12 has a top surface thatincludes an upwardly facing annular groove 20. The upwardly facingannular groove 20 is dimensioned to receive a metal ring gasket 30 inaccordance with the invention. The second subcomponent 14 has a bottomsurface that includes a downwardly facing annular groove 22. Theupwardly facing and downwardly facing annular grooves 20, 22 mate whenthe second subcomponent 14 is connected to the first subcomponent 12 todefine a hexagonal annular cavity 24. However, as will be explainedbelow, the annular cavity 24 need not necessarily be hexagonal toprovide the energized high-pressure fluid seal in accordance with theinvention.

As shown in FIG. 1, the second subcomponent 14 is secured to the firstsubcomponent 12 by a threaded nut 40. The threaded nut 40 has boxthreads 42 for engaging pin threads 44 formed externally on the firstsubcomponent 12. In one embodiment, the threaded nut 40 is a wing nutand includes a plurality of lugs 46 that extend radially from a mainbody 48 of the threaded nut 40. The lugs 46 have impact surfaces 46 awhich may be impact-torqued using a hammer or mallet (not shown) in theusual way in which a hammer union is “hammered up”. In anotherembodiment, the threaded nut 40 is a “spanner nut” that includes flats,bores, or the like, that are gripped by a spanner wrench (not shown) topermit the threaded nut 40 to be tightened to a required torque. As willbe understood by those skilled in the art, the wrench used to tightenthe nut may be a torque wrench, which indicates the torque applied tothe threaded nut 40 to ensure that it is tightened with a precise amountof torque.

The threaded nut 40 in accordance with this embodiment of this inventionis constructed in three parts so that a main body of the nut 40 can be asingle piece construction for greater strength. As is understood bythose skilled in the art, the nuts for hammer unions are commonly cutinto two parts that are welded together in situ after the nut ispositioned above an annular shoulder 14 a of the second subcomponent 14.However, this compromises the holding strength of the nut, which isstrained when the hammer union is exposed to very high fluid pressure.The threaded nut 40 in accordance with the invention has an upperannular shoulder 47 that extends radially inwardly from a top of themain body 48 of the nut 40. The annular shoulder 47 abuts a flange 52that extends radially outwardly from an adapter collar 50. The adaptercollar 50 is a generally annular multi-piece body having an innerdiameter dimensioned to slide over an outer surface of the secondsubcomponent 14 until a bottom surface 54 of the adapter collar 50 abutsthe annular shoulder 14 a of the second subcomponent 14. A bottomsurface of the annular shoulder 14 a, in turn, abuts a top surface 16 ofthe first subcomponent 12. When torque is applied to the nut 40, theupper annular shoulder 47 of the nut 40 is forced downwardly on theflange 52, which in turn exerts a downward force on the annular shoulder14 a, thereby forcing the bottom surface 18 of the second component 14against the top surface 16 of the first subcomponent 12, and thusforcing the metal ring gasket 30 to a set position in the annular cavity24. In one embodiment, the multi-piece adapter collar 50 is constructedof two symmetrical parts.

As further shown in FIG. 1, the adapter collar 50 includes an annulargroove 56 dimensioned to receive an inner edge of a segmented retainerplate 60. The segmented retainer plate 60 is secured to a top of the nut40 by threaded fasteners 62, which are received in a plurality of tappedbores 49 distributed in a circular pattern around a top of the nut. Inone embodiment, the segmented retainer plate 60 is constructed of threewedge-shaped pieces.

In the embodiment shown in FIG. 1, the metal ring gasket 30 provides ahigh-pressure metal-to-metal seal between the first and secondsubcomponents 12, 14. The threaded union 10 also includes a pair ofelastomeric backup seals, e.g. O-rings, which are seated in annulargrooves 70 in the second subcomponent. Alternatively, the annulargrooves 70 could be machined into the first subcomponent. It will beappreciated that the number of elastomeric annular sealing elements canbe varied from zero to three or more.

FIG. 2 illustrates, in an exploded view, the threaded union 10 shown inFIG. 1. As shown in FIG. 2, the threaded union 10 includes a pair ofO-rings 80, each having its own backing member 82. The O-rings 80 andbacking members 82 are dimensioned to be received in each of the twoannular grooves 70 in order to provide the elastomeric backup seal tothe metal-to-metal seal provided by the metal ring gasket 30.

FIG. 3 illustrates a threaded union 10 in accordance with anotherembodiment of the invention. The high-pressure fluid-tight seal betweenthe first and second subcomponents 12, 14 is provided only by the metalring gasket 30. Otherwise, the embodiments shown in FIGS. 2 and 3 areidentical.

In testing, the metal ring gasket 30 has maintained a fluid-tight sealup to a fluid pressure of 30,000 psi. The metal ring gasket is also ableto maintain a high-pressure seal even if exposed to elevatedtemperatures due to fire.

As illustrated in FIG. 4, one embodiment of the invention metal ringgasket 30 has beveled corners (or beveled surfaces) and an octagonalcross-section. In one embodiment, the corners of the metal ring gasketare beveled at an angle of 23°±1°. Persons skilled in the art willappreciate that the bevel angle may be changed within limits withoutaffecting the efficacy of the energized seal. The metal ring gasket 30is preferably made of steel. Plain carbon steel or stainless steel isselected depending on whether a fluid to be contained is corrosive ornon-corrosive.

For service where corrosion is not generally problematic AISI 1018nickel-plated cold-drawn steel may be used. The AISI 1018 steel has acarbon content of 0.18% (although it may vary from 0.14% to 0.20%), amanganese content of 0.6% to 0.9%, a maximum phosphorus content of 0.04%and a maximum sulfur content of 0.05%. The AISI 1018 steel exhibits highmachinability (its average machinability rating is 70%), good fracturetoughness, good surface hardness (126 HB), high tensile strength (440MPa), high yield strength (370 MPa), superior ductility (40-50%reduction in cross-sectional area at the fracture load) and isrelatively inexpensive. Alternatively, other plain carbon steels may besubstituted, provided they have approximately similar mechanicalproperties.

For service where corrosion is problematic the metal ring gasket may bemade using either AISI 316 stainless steel or AISI 304 stainless steel.Not only are these stainless steels corrosion-resistant but they alsopossess desirable mechanical properties (in terms of machinability,fracture toughness, surface hardness, tensile strength and yieldstrength).

Alternatively, persons skilled in the art will appreciate that, forcertain applications, the metal ring gaskets in accordance with theinvention may be made using metals other than steel (such as aluminum orcopper alloys like brass or bronze, for example), which are moretemperature-resistant than elastomeric gaskets.

As illustrated schematically in FIG. 4, when the threaded nut 40 istightened, the nut 40 exerts a force F on the first and secondsubcomponent 12,14. The forces F elastically deforms the metal ringgasket 30 within the annular cavity 24. The compressive force Fc actingon the outer beveled surfaces can be expressed by the equation: F_(C)=Fsin 23°, assuming a bevel angle of 23°.

FIG. 4 shows the undeformed metal ring gasket 30 in substantiallyunloaded contact with the inner beveled surfaces of the annular cavity24, for example immediately prior to or immediately after torquing ofthe threaded nut 40. When the threaded nut 40 has been tightened to theextent shown in FIG. 4, the annular cavity 24 has a hexagonal crosssection with internal beveled surfaces, or facets, that have angles thatcorrespond to the bevel angles of the octagonal cross section of themetal ring gasket 30. As shown in FIG. 4, the top, bottom and inner sidesurfaces of the metal ring gasket do not contact the top, bottom orinner surfaces of the annular cavity 24. In other words, there remainsat all times, even after full torgue, an upper gap G_(U) between the topsurface of the metal ring gasket 30 and the top (inner) surface of theannular cavity 24. Likewise, there remains at all times a lower gapG_(L) between the bottom surface of the metal ring gasket 30 and thebottom (inner) surface of the annular cavity 24, a gap G_(S) between theinner side of the metal ring gasket 30 and the inner side of the annularcavity 24; and, a gap G_(B) between the inner beveled corners of themetal ring gasket 30 and the annular cavity 24.

When the forces F_(C) act on each of the outer beveled corners the metalring gasket 30 the forces cause the metal ring gasket 30 to beelastically deformed inwardly to provide a fluid-tight seal between thefirst and second subcomponents.

As schematically illustrated in FIG. 5, the metal ring gasket isover-sized to have an outer diameter such that the outer beveled cornersmust be displaced by elastic deformation of the metal ring gasket 30 byabout 0.003″ when the first subcomponent 12 and the second subcomponent14 are securely interconnected. It should be noted that in FIG. 5 theoversizing of the metal ring gasket is shown at an exaggerated scale forthe purposes of illustration. The 0.003″ oversizing is consideredoptimal for the steels described above because the metal ring gasket 30is elastically, and not plastically deformed in the annular cavity 24.It will be appreciated that for other steels, and/or for other sizes ofthreaded unions, the oversizing may be different to provide an optimalseal. As further shown in FIG. 5, an inner diameter of the metal ringgasket 30 is larger than a diameter of the annular cavity 24. In oneembodiment the inner diameter of the metal seal ring is such that thegap G_(B) is at least 0.001″ after the metal ring gasket 30 iselastically deformed in the annular groove 24. In one embodiment, theinner diameter of the metal ring gasket 30 is about 0.014″ larger thanan inner diameter of the annular gap 24 before the metal ring gasket iselastically deformed. However, this difference in the diameters is notcritical and can be varied considerably, so long as the metal ringgasket 30 can be elastically deformed without the elastic deformationbeing inhibited by contact with an inner face of the annular cavity 24.Consequently, if an inner diameter of the metal ring gasket is at least0.003″ larger than an inner diameter of the annular cavity 24, the metalring gasket can be elastically deformed as required.

In operation, the threaded union 10 is torqued or “hammered up” bytightening the nut 40 until the end surfaces 16,18 of the first andsecond subcomponents 12,14 abut. Due to the slight over-sizing (about0.003″) of the metal ring gasket 30, the threaded union cannot beovertorqued, and there is no danger of plastic deformation of the metalring gasket 30. The metal ring gasket 30 can therefore be repeatedlyreused so long as the sealing surfaces on its beveled faces are notscratched or marred.

FIG. 6 schematically illustrates the threaded union 10 when it isexposed to elevated fluid pressures. As is understood in the art, highfluid pressures in the fluid passage 15 (FIG. 3) force the end surfaces16,18 of the first and second subcomponents 12,14 apart due to elasticdeformation of the threaded nut 40. This creates a gap 90 between thefirst and second subcomponents 12,14. Fluid pressure flows through thegap 90 on the inner side of the metal ring gasket 30. The fluid pressureinduces hoop stress in the metal ring gasket 30 that forces the sealingsurfaces 30 a, 30 b of the metal ring gasket 30 into tighter contactwith the corresponding surfaces of the annular cavity 24, and the sealis “energized” Consequently, the higher the fluid pressure (within thepressure capacity of the first and second subcomponents 12,14) in thecentral fluid passage 15, the more energized and tighter the fluid seal.

FIG. 7 schematically illustrates another embodiment of a metal ringgasket 32 in accordance with the invention. The metal ring gasket 32 hasan outer diameter and outer beveled corners that are configured the sameas described above with reference to FIGS. 4-6 and the metal ring gasketis elastically deformed in the annular cavity 24 in the same way.However, the metal ring gasket 32 is hexagonal in cross-section and hasa flat inner side that is spaced from an inner surface of the annularcavity 24. The fluid pressure acts on the flat side to energize the sealas described above. As will be understood by those skilled in the art,the shape of the inner side, and consequently, the cross-sectional shapeof the metal ring gaskets 30,32 is a matter of design choice.

The threaded union 10 in accordance with the invention may be used toconstruct a high-pressure, fluid-tight seal between a drilling flange,described in applicant's co-pending U.S. patent application Ser. No.10/656,693 filed Sep. 4, 2003 and a wellhead on a wellhead assembly, asdescribed and illustrated in applicant's co-pending U.S. patentapplication Ser. No. 10/690,142 (Dallas) entitled METAL RING GASKET FORA THREADED UNION, which are hereby incorporated by reference, as well asa fluid conduit for any other application.

The metal ring gasket in accordance with the invention has beenextensively pressure-tested in a number of threaded unions integratedinto different wellhead and well stimulation tool components. It hasproven to be extremely reliable and provides a very high-pressureenergized seal that is easy to “torque up” using a hammer or a wrench.This permits such components to be more economically constructed andmore quickly assembled. Cost savings are therefore realized, whileworker safety and environmental protection are ensured.

As will be understood in the art, the metal ring gasket 30,32 for thethreaded union 10 can be used in a variety of applications to reducecost, while ensuring high performance and safety in fluid conduits ofall types, including wellhead assemblies and well stimulation equipment,where very high pressure and very high temperature resistance areespecially important.

The embodiments of the invention described above are therefore intendedto be exemplary only. The scope of the invention is intended to belimited solely by the scope of the appended claims.

1. A threaded union for providing a high-pressure, fluid-tight,metal-to-metal seal in a fluid conduit, the threaded union comprising:first and second subcomponents that are interconnected by a nut, thefirst and second subcomponents having respective mating ends withcomplementary ring gasket grooves therein that form an annular cavitywhen the mating ends abut; and a metal ring gasket received in theannular cavity, wherein the metal ring gasket has an outer diameter thatis slightly larger than an outer diameter of the annular cavity and themetal ring gasket is elastically deformed in the annular cavity toprovide a high-pressure fluid-tight seal between the first and secondsubcomponents when the first and second subcomponents are securelyinterconnected by the nut.
 2. The threaded union as claimed in claim 1wherein the outer diameter of the metal ring gasket comprises beveledsurfaces angled to mate with complementary surfaces on an outer diameterof the annular grooves, and there is a gap between an inner side of themetal ring gasket and an inner wall of each of the annular grooves whenthe first and second subcomponents are securely interconnected by thenut.
 3. The threaded union as claimed in claim 2 wherein the outerdiameter of the metal ring gasket is about 0.003″ larger than an outerdiameter of each annular groove at the respective mating surfaces. 4.The threaded union as claimed in claim 3 wherein the gap between themetal ring gasket and the inner walls of the annular grooves is at least0.003″.
 5. The threaded union as claimed in claim 3 wherein top andbottom surfaces of the metal ring gasket are spaced apart fromrespective upper and lower surfaces of the annular cavity, therebydefining upper and lower gaps, respectively, when the first and secondsubcomponents are securely interconnected by the nut.
 6. The threadedunion as claimed in claim 1 wherein the threaded nut is constructed inthree parts so that a main body of the nut can be a single piececonstruction for greater strength.
 7. The threaded union as claimed inclaim 6 wherein the threaded nut has an upper annular shoulder thatextends radially inwardly from a top of the main body of the nut, theannular shoulder abuts a flange that extends radially outwardly from anadapter collar that is a generally annular multi-piece body having aninner diameter dimensioned to slide over an outer surface of the secondsubcomponent until a bottom surface of the adapter collar abuts anannular shoulder of the second subcomponent, and when torque is appliedto the nut the upper annular shoulder is forced downwardly on the flangewhich in turn exerts a downward force on the annular shoulder, therebyforcing the bottom surface of the second component against a top surfaceof the first subcomponent to compress the metal ring gasket into theannular grooves.
 8. A metal ring gasket for providing a high-pressure,fluid-tight metal-to-metal seal in an annular cavity formed by annulargrooves at an interface of first and second subcomponents of a threadedunion, the metal ring gasket comprising a generally annular body havingbeveled outer corners for receiving compressive loads exerted on themetal ring gasket by complementary surfaces on an outer diameter of theannular cavity when the first and second subcomponents are securelyinterconnected, the metal ring gasket having an outer diameter that isslightly larger than an outer diameter of the annular cavity and themetal ring gasket is elastically deformed in the annular cavity toprovide a high-pressure fluid-tight seal between the first and secondsubcomponents when the first and second subcomponents are securelyinterconnected by the nut.
 9. The metal ring gasket as claimed in claim8 wherein an inner side of the metal ring gasket is spaced away from aninner side of the annular cavity when the first and second subcomponentsare securely interconnected by the nut.
 10. The metal ring gasket asclaimed in claim 8 wherein the beveled outer corners and thecomplementary surfaces are oriented at an angle of about 23° withrespect to a plane of end faces of the first and second subcomponents.11. The metal ring gasket as claimed in claim 10 wherein the metal ringgasket is octagonal in cross-section.
 12. The metal ring gasket asclaimed in claim 10 wherein the metal ring gasket is hexagonal incross-section and an inner side of the metal ring gasket is flat. 13.The metal ring gasket as claimed in claim 8 wherein pressurized fluidswithin the first and second subcomponents induce hoop stress in themetal ring gasket that forces the beveled corners into closer contactwith the mating surfaces to energize the high-pressure, fluid-tight,metal-to-metal seal.
 14. The metal ring gasket as claimed in claim 8further comprising top and bottom surfaces that are spaced apart byupper and lower gaps, respectively, from the upper and lower surfaces ofthe annular groove.
 15. The metal ring gasket as claimed in claim 8wherein the metal ring gasket is made of a metal having a ductilitywhich exhibits at least 40% reduction in cross-sectional area at afracture load.
 16. The metal ring gasket as claimed in claim 15 whereinthe metal comprises a ductile carbon steel for non-corrosive service.17. The metal ring gasket as claimed in claim 15 wherein the metalcomprises a ductile stainless steel for corrosive service.
 18. A methodof providing a high-pressure fluid-tight seal between first and secondsubcomponents of a threaded union, the method comprising: determining aninner diameter and an outer diameter of an annular metal ring gasketgroove in a mating face of the first and second subcomponents; andmanufacturing a metal ring gasket to be received in an annular cavityformed by the respective metal ring gasket grooves when the first andsecond subcomponents are securely interconnected by a nut of thethreaded union, the metal ring gasket having outer faces for matingcontact with complementary faces in the respective metal ring gasketgrooves, an outer diameter that is slightly larger than an outerdiameter of the annular ring gasket grooves and an inner diameter thatis slightly larger than an inner diameter of the ring gasket grooves sothat the metal ring gasket is elastically deformed when placed in theannular grooves and the first and second subcomponents are securelyinterconnected, but a gap remains between an inner side of the metalring gasket and an inner surface of the annular cavity.
 19. The methodas claimed in claim 18 wherein the manufacturing comprises manufacturingthe metal ring gasket with an outer diameter such that the outer facesfor mating contact with the complementary faces are respectivelydisplaced by about 0.003″ by elastic deformation of the metal ringgasket when the first and second subcomponents are securelyinterconnected.
 20. The method as claimed in claim 18 wherein themanufacturing comprises manufacturing the metal ring gasket with aninner diameter such that the inner side of the metal ring gasket isspaced at least 0.001″ from an inner side of the annular cavity when thefirst and second subcomponents are securely interconnected.